BIOCONTROL OF β – HAEMOLYTIC AEROMONAS HYDROPHILA INFECTION IN LABEO ROHITA USING ANTAGONISTIC BACTERIUM PSEUDOMONAS AERUGINOSA FARP72

Received on 20 September, 2013; received in revised form, 23 October, 2013; accepted, 11 January, 2014; published 01 February, 2014

BIOCONTROL  OF  β - HAEMOLYTIC AEROMONAS  HYDROPHILA INFECTION  IN  LABEO  ROHITA  USING  ANTAGONISTIC BACTERIUM  PSEUDOMONAS  AERUGINOSA  FARP72

Farhana Hoque

Department of Aquatic Animal Health, Faculty of Fishery Sciences, West Bengal University of Animal and Fishery Sciences, 5 -Budherhat Road, Chakgaria, P.O. Panchasayar, Kolkata -700094,West Bengal, India

ABSTRACT: Pseudomonas aeruginosa FARP72 was isolated from the skin mucus of freshwater catfish (Clarias batrachus) as a potent antagonistic bacterium. Efforts have made to assess the antagonistic activity against potential fish and human bacterial pathogens in vitro. Antagonistic effects were confirmed by cross-streaking method against pathogenic bacteria such as Aeromonas hydrophila, Bacillus subtilis, Bacillus amylolequifasciens, Escherichia coli, Edwardsiella tarda, Klebsiella pneumoniae, Enterohaemorrhagic Escherichia coli (EHEC), Pseudomonas aeruginosa, Salmonella Typhi, beta haemolytic Streptococcus, Staphylococcus aureus ATCC12598and Vibrio cholera. The LD₅₀ value of Pseudomonas aeruginosaFARP72 and β-haemolytic Aeromonas hydrophila N₁₀P were calculated to be >5.65×10¹⁰ cells/ fish and 2.37×10⁸ cells/fish, which proved them to be non-pathogenic and moderate pathogenic respectively. In biocontrol experiment, the Labeo rohita stocks which received a first 15 min dip in Pseudomonas aeruginosaFARP72 suspension containing 1x10⁷cells/ml followed by another dip in β- haemolytic Aeromonas hydrophila suspension containing 1x10⁵ cells/ml for 15 min and subsequently cohabited with healthy Labeo rohita showed 37% mortality; while 75% of the fish died when treated only with β- haemolytic Aeromonas hydrophila suspension containing 1x10⁵ cells/ml in 30 days of post-challenge. The results suggest that Pseudomonas aeruginosaFARP72 can be a substitute to the commercially available and indiscriminately used common antibiotics, reducing the attachment of Aeromonas hydrophila and development of infection and disease outbreaks. Judicious use would help overcome the problems of developing antibiotic resistant pathogenic strains, side effects of antibiotics etc. without causing any environmental pollution, reducing stress and better disease management and ensuring its practical approach for betterment of animal life.

Antagonistic bacteria,Fish and human bacterial pathogens, LD₅₀ value, Biocontrol

INTRODUCTION:The world aquaculture production of food fish is increased by 6.2% from 59 million tonnes in 2010 to 62.7 million tonnes in 2011 ¹.  

To offer high productivity intensive farming has become the best option for the aqua-farmers. Invariably, the high stocking density and high feed input came in to play which results in greater amount of feed wastage, organic build up and accompanies several disease problems, in turn, decreases the growth rate, and the high organic load stimulates proliferation of several opportunistic pathogens ^{2, 3}. Due to this negative balance, aquaculturists often face mass mortality or disease occurrence of their stocks.

The bacteria Aeromonas hydrophila is a widely distributed pathogenic bacteria especially in warm water throughout the world causing MAS (motile Aeromonas septicemia)  and epizooticulcerative syndrome (EUS) of different fishes both farmed and wild ^4,5. The use of antimicrobials and indiscriminate use of the commercial antibiotics in disease prevention can bring about the emergence of drug-resistant microorganisms and leave antibiotic residues in the body and in the environment; chemotherapy may kill or inhibit the normal micro flora in the digestive tract which is beneficial to the health. Therefore, alternative strategies to prevent opportunistic infections in aquaculture are strongly needed. The use of antagonistic bacteria to control potential pathogens is an alternative method and is gaining acceptance ^{6, 7}.

Keeping this in view, the present study was taken up with the following objectives:  Isolation of antagonistic bacterium from freshwater fish skin mucus and assess the inhibitory activity against selected human and fish bacterial pathogens in-vitro, to test the pathogenicity of the potent antagonistic bacterium and pathogenic bacterial strain isolated from diseased fish and to assess the efficacy of potent antagonistic bacteriumas biocontrol agent to inhibit the fish bacterial pathogen, Aeromonas hydrophila in-vivo.

MATERIALS AND METHODS:

 Experimental fish for bacterial challenge and biocontrol assays: The experimental fish Labeo rohita of two size groups, viz., 12.02±0.94 cm and 21.21±5.01g, and 3.89±0.69 cm and 2.67±0.71g were procured from commercial fish breeders of Naihati (Lat 22°54’10” N and Long 88°25’01’’ E), North 24 Parganas district, West Bengal for bacterial challenge (pathogenicity) and biocontrol assays, respectively. The fish, on receipt, were disinfected with 5 ppm potassium permanganate (KMnO₄) solution and acclimatized for 10 days in circular FRP tanks of 500 L capacity containing bore-well water with continuous aeration. All fishes were fed a balanced pellet diet at the rate of 2% of their body weight and maintained under optimal condition.

Pathogenic bacterial strains: The details of human and fish bacterial pathogens used in the study and their sources are presented in Table 1.

TABLE 1: DETAILS OF THE HUMAN AND FISH BACTERIAL PATHOGENS AND THEIR SOURCES

Antagonistic bacterial strain Pseudomonas aeruginosa FARP72: Total 41 potent antagonistic bacteria were isolated, purified and identified from 25 fresh water fish samples. Out of which the broad spectrum of inhibitory activity was elucidated byGram negative Pseudomonas aeruginosa FARP72 isolated from Clarias batrachus.

Inhibitory activity of antagonistic bacterium Pseudomonas aeruginosa FARP72 against fish and human pathogens in-vitro: In-vitroinhibitory activity of the antagonistic bacterium Pseudomonas aeruginosa FARP72from fish mucus was tested against eighteen opportunistic bacterial human and fish pathogens by modified cross streak technique ^{8, 9}. The severity of inhibition was determined by parallel streaking method ¹⁰. The inhibitory activity of the producer (antagonistic) strain was determined on the basis of growth inhibition of the test strain and the severity of inhibition was rated as “++” for total inhibition, “+” for feeble / thin growth  and “–“ for no inhibition and comparison with control plates.

Pathogenicity and biocontrol experiments

Preparation of bacterial cell suspensions of Pseudomonas aeruginosa FARP72 and β- haemolytic Aeromonas hydrophila N₁₀P: Antagonistic Pseudomonas aeruginosa FARP72 and pathogenic β-haemolytic Aeromonas hydrophila N₁₀P (collected from diseased fish sample),maintained on Tryptone soya agar (TSA) slant was streaked on to a TSA plate and incubated at 30±2^oC for 24 h to get young discrete colonies. One or two young colonies were aseptically picked, transferred to 10 ml TSB and incubated at 30±2^oC for 24 h. This 24 h old culture was then transferred to 300 ml Tryptone soya broth (TSB) and reincubated at 30±2^oC for 48 h. The cells were harvested by centrifugation at 7500 rpm for 20 min at 25^oC in a cooling centrifuge (Remi, Mumbai). The cell pellets were washed twice by centrifugation with sterile physiological saline and finally resuspended in 30 ml sterile physiological saline and used immediately. A portion of the cell suspension was suitably diluted up to 10^-9 in sterile saline and the number of cells/ml of suspension was determined by drop count method ¹¹ on TSA after incubation at 30±2^oC for 48 h.

Determination of water quality parameters: The water samples for determining water quality parameters were collected in glass containers of 300 ml capacity. The pH, temperature, dissolved oxygen (DO), conductivity, total dissolved solids (TDS) and ammonia were measured by Hanna Multi-parameter water analysis kit, Ecuador and Thermo Scientific multi-parameter water analysis instrument, Beverly, USA.

Experimental set up:             All the experiments were carried out in the wet laboratory of the Department of Aquatic Animal Health. The ambient temperature was generally in the range of 28 – 36^oC and the laboratory was well ventilated so as to allow sufficient fresh air. All the experimental fish were first disinfected by placing in 5 ppm KMnO₄ solution for 15 min. The morbid fish and morts were removed immediately and the healthy ones were stocked at the rate of 100 fish/tank in circular FRP tanks of 500 L capacity containing aged bore-well water for 10 days with continuous aeration. All fish were fed a balanced basal dry pellet feed twice daily at the rate of 2% of their body weight and maintained under optimal condition. The wastes and faecal matter were siphoned out twice in a week and 50% water exchange was done in three days interval.

Bacterial challenge tests: The growth conditions provided to the experimental fish during challenge tests are given in Table 2.

TABLE 2 - GROWTH CONDITIONS PROVIDED TO THE EXPERIMENTAL FISH DURING CHALLENGE TESTS

Challenge test with antagonistic bacterium, Pseudomonas aeruginosa FARP72: The antagonistic Pseudomonas aeruginosaFARP72 strain at levels of 5.65x10¹⁰ cells/fish, 5.65x10⁹ cells/fish, 5.65x10⁸ cells/fish and 5.65 x10⁷ cells/fish were used in experimental challenge through intramascular injection ¹² .Twelve glass aquaria (60 cm L x 45 cm B x 30 cm H) of 50 L capacity were used for the experiment. Prior to use, the glass aquaria were scrubbed, cleaned with chlorinated water (200 ppm), flushed thoroughly with freshwater, dried for 5 days before use and arranged in three tier iron stands. The glass aquaria were grouped in to six groups namely A, B, C, D, E and F with two aquaria for each group as given below. All glass aquaria were filled with clean water to a volume of 30 L each and conditioned for three days.

After three days, the glass aquaria were stocked with experimental fish at the rate of 10 fish/aquarium tank from the acclimatized stocks. The aquaria were labelled and covered with nylon netting for adequate protection. The fishes were acclimatized in aquaria for three days. After acclimatization in the glass aquaria, each fishes were injected with 0.1ml of bacterial suspension at different concentrations as required. Positive control fish received 0.1ml of 0.85% physiological saline instead of bacterial suspension. Negative control fishes received no injection. The challenged fishes of each group were maintained in the respective aquaria for 20 days. Mortality, external signs of infection and behavioural abnormalities were recorded daily. The experiment was carried out in duplicate. The median lethal dose (LD₅₀) was calculated ¹³.

EXPERIMENTAL SETUP FOR CHALLENGE TEST WITH ANTAGONISTIC BACTERIUM, PSEUDOMONAS AERUGINOSA FARP72:

Group A: Fish challenged with 10¹⁰cells/fish; Group B: Fish challenged with 10⁹cells/fish; Group C: Fish challenged with 10⁸cells/ fish; Group D: Fish challenged with 10⁷cells/ fish; Group E: Fish injected with 0.85% physiological saline and served as positive control; Group F: Fish received no injection and served as negative control.

Challenge test with pathogenic β-haemolytic Aeromonas hydrophila N₁₀P: The fish pathogenic β-haemolytic Aeromonas hydrophila at levels of 6.37x10¹⁰ cells/fish, 6.37x10⁹ cells/fish, 6.37x10⁸ cells/fish and 6.37x10⁷ cells/fish were used in experimental challenge through intramascular injection ¹². The experimental design and protocol are as described in previous section.

Pathogenicity test by Cohabitation challenge:

Cohabitation challenge: In order to examine the water-borne route for transmission of Pseudomonas aeruginosa FARP72 and β- haemolytic Aeromonas hydrophila from infected to non-infected fish, cohabitation challenge experiments ¹⁴ were done using ‘Skin wounded’ and ‘Non-injured’ fish (2.67±0.71g; 10 fish/treatment in duplicate) as recipients. Eight glass aquaria (60 cm L x 45 cm B x 30 cm H) of 50 L capacity were used for the experiment. The glass aquaria were scrubbed, cleaned with chlorinated water (200 ppm), flushed thoroughly with freshwater and dried for 5 days before use. The glass aquaria were grouped in to four groups namely A, B, C and D with two aquaria for each group as given below. All glass aquaria were filled with clean water to a volume of 30 L each and conditioned for three days. After three days, the glass aquaria were stocked with experimental fish at the rate of 50 fish/ aquarium tank from the acclimatised stocks. The aquaria were labelled and covered with nylon netting for adequate protection. The fishes were acclimatised in the aquaria for three days. After acclimatisation in the glass aquaria, scales of ten fishes from each aquarium were scrapped off gently with a scalpel from caudal peduncle to the pectoral fin, i.e., in the opposite direction (skin wounded).

Ten fishes from each of the first two aquaria were dipped in β-haemolytic Aeromonas hydrophila suspension containing 1x10⁵ cells/ml for 15 min (Group A). Similarly the ten fishes from each of the other two aquaria were dipped in Pseudomonas aeruginosaFARP72 suspension containing 1x10⁷ cells/ml for 15 min (Group B).

Ten skin wounded fish received a 15 min dip in 0.85% physiological saline instead of bacterial suspension and served as positive control (Group C). All the fish after dip treatment were transferred to the respective aquarium containing 40 healthy (non-injured) fish. Negative control fish (all non-injured) were kept undisturbed (Group D). One day before the treatment and on the day of treatment the fish were starved and 24 h post treatment fish were fed at the rate of 2% of body weight twice daily.

EXPERIMENTAL SETUP FOR COHABITATION CHALLENGE:

Group A: Ten skin wounded fish dipped in β-haemolytic Aeromonas hydrophila suspension, 1x10⁵ cells/ml for 15 min and introduced in to the respective aquarium containing healthy fish; Group B: Ten skin wounded fish dipped in Pseudomonas aeruginosaFARP72 suspension, 1x10⁷cells/ml for 15 min and introduced in to the respective aquarium containing healthy fish; Group C: Ten skin wounded fish dipped in 0.85% physiological saline for 15 min and introduced in to the respective aquarium containing healthy fish. This served as positive control; Group D:  Non injured fish received no dip and served as negative control.

External gross and clinical signs of infections, behavioural abnormalities and mortality were recorded daily. Samples from the affected haemorrhagic parts of the body of freshly dead fish were taken and inoculated on to glutamate starch phenol red agar (Hi-Media, Mumbai, India) plates or Pseudomonas isolation agar and then incubated at 30±2°C for 24 h. Standard biochemical tests were performed to identify whether or not the infective agent is pre-inoculated β- haemolytic Aeromonas hydrophila N₁₀P or Pseudomonas aeruginosa FARP72.Cumulative mortality was computed from the daily mortality data.

Biocontrol Experiment: Six glass aquaria (60 cm L x 45 cm B x 30 cm H) of 50-L capacity were used for the experiment. The glass aquaria were scrubbed, cleaned with chlorinated water (200 ppm), flushed thoroughly with freshwater and dried for 5 days before use. The glass aquaria were grouped in to five groups namely A, B, C, D and E with two aquaria for each group as given below. All glass aquaria were filled with clean water to a volume of 30-L each and conditioned for three days. After three days, the glass aquaria were stocked with experimental fish (2.67±0.71g) at the rate of 50 fish/ aquarium tank from the acclimatized stocks. The aquaria were labelled and covered with nylon netting for adequate protection. The fishes were acclimatized in the aquaria for three days. After acclimatization in the glass aquaria, scales of ten fishes from each aquarium were scrapped off gently with a scalpel in from caudal peduncle to the pectoral fin, i.e., in the opposite direction. Ten fishes from each of the first two aquaria were dipped in β-haemolytic Aeromonas hydrophila suspension containing 1x10⁵ cells/ml for 15 min (Group A).

Ten skin wounded fishes from each of the two aquaria were received a first 15 min dip in Pseudomonas aeruginosaFARP72 suspension containing 1x10⁷cells/ml followed by another dip in β- haemolytic Aeromonas hydrophila N₁₀P suspension containing 1x10⁵ cells/ml for 15 min (Group B). Positive control skin wounded fish received a 15 min dip in 0.85% physiological saline instead of bacterial suspension (Group C). Negative control non-injured fish were kept undisturbed (Group D). The challenged fishes of each group were maintained in the respective aquaria for 30 days. All fish were fed a balanced basal dry pellet feed twice daily at the rate of 2% of their body weight.

One day before the treatment and on the day of treatment the fish were starved. External signs of infections, behavioural abnormalities and mortality were recorded daily. The experiment was carried out in duplicate. Cumulative mortality was computed from the daily mortality data.

SETUP FOR BIOCONTROL EXPERIMENT:

Group A: Ten skin wounded fish dipped in β- haemolytic Aeromonas hydrophila N₁₀P suspension, 1x10⁵ cells/ml for 15 min and introduced in to the respective aquarium containing healthy fish; Group B: Ten skin wounded fish dipped in Pseudomonas aeruginosaFARP72 suspension containing 1x10⁷cells/ml for 15 min and introduced in to the respective aquarium containing healthy fish; Group C: Ten skin wounded fish first dipped in Pseudomonas aeruginosaFARP72 suspension containing 1x10⁷cells/ml for 15 min and followed by dip in β- haemolytic Aeromonas hydrophila N₁₀P suspension containing 1x10⁵ cells/ml for 15 min and introduced in to the respective aquarium containing healthy fish; Group D: Ten skin wounded fish dipped in 0.85% physiological saline for 15 min and introducted in to the respective aquarium containing healthy fish. This served as positive control; Group E: Non injured fish received no dip and served as negative control.

Statistical analysis: The results of the bacterial counts were processed by log transformation. One way analysis of variance (ANOVA) was followed to test the level of significance among the log transformed bacterial counts of different fish species and locations. Students’‘t’ test was used to determine the significant differences in fish mortality among treatments.

RESULTS AND DISCUSSION:

Antibacterial spectrum of Pseudomonas aeruginosa FARP72 against human and fish pathogens: Pseudomonas aeruginosa FARP72 was proven to be a potent antagonistic bacterium on the basis of the broad spectrum of activity and zone of inhibition (>5 mm) produced against the 18 test strains by cross streaking method (Table 3).

In parallel streaking method, it was observed that the growth of Pseudomonas aeruginosa CNMC; Escherichia coli  CNMC, Edwardsiella tarda SDDL; Staphylococcus aureus ATCC 12598 Vibrio cholerae FCRI; Enterohaemorrhagic Escherichia coli FCRI; Salmonella Typhi FCRI; Aeromonas sp. AAH; Pseudomonas sp. FPT; Bacillus pumilus FPT; Bacillus amyloliquefaciens FPT, Staphylococcus aureus S2,FPT; Escherichia coli FPT; β-haemolytic Streptococcus sp. AAH; β- haemolytic Aeromonas hydrophila AAH was totally inhibited; whereas Klebsiella pneumoniae CNMC and Staphylococcus aureus CNMC were only slightly inhibited by Pseudomonas aeruginosa FARP72 strain and represented in Table 3.

Pathogenicity and biocontrol experiment:

Challenge test with Pseudomonas aeruginosa FARP72: The results of the pathogenicity test of Pseudomonas aeruginosaFARP72 on Labeo rohita and the respective LD₅₀ valueare presented in Table 4. The first mortality was observed in 24 h post-injection. Within 24 h, one fish died in the tank containing 10 fish challenged with 5.65x10¹⁰ cellsof Pseudomonas aeruginosaFARP72/fish, another two fish died after 48 h of injection in the same stock.

The stocks where Pseudomonas aeruginosaFARP72 were injected at a level of 5.65x10⁹ cells/fish, 5.65x10⁸ cells/fish, 5.65x10⁷ cells/fish remained active with no serious abnormalities and mortalities except the primary inflammatory response at the place of injection, which was recovered in 3-4 days post-injection period. No mortalities were noticed in stocks which were used as positive control and negative control. After 15 days, 30% fish mortalities were observed in the stock inoculated with 5.65x10¹⁰ cells/fish. The lethal dose at which 50% of the experimental population die (LD₅₀) was calculated on the basis of mortality.

TABLE 3: ANTIBACTERIAL SPECTRUM OF PSEUDOMONAS AERUGINOSA FARP72 AGAINST HUMAN AND FISH PATHOGENS BY CROSS STREAK AND PARALLEL STREAK METHODS IN-VITRO

+: Feeble/Thin growth, ++: Total inhibition.

CNMC-Calcutta National Medical College and Hospital, Kolkata; SDDL –Shrimp Disease Diagnostic Laboratory, Tamil Nadu Fisheries University, Madhavaram, Chennai; FCRI - Fisheries College and Research Institute, Thoothukudi; AAH - Department of Aquatic Animal Health, FFSc, Kolkata; FPT- Department of Fish Processing Technology, FFSc, Kolkata

The LD₅₀ value of Pseudomonas aeruginosaFARP72 was calculated to be >5.65×10¹⁰ cells/ fish (Table 3). Samples from the affected parts of the body of freshly dead fish yielded translucent and colourless colonies initially on Pseudomonas isolation agar (Hi-Media, Mumbai, India) plates with 24 h of incubation at 30±2°C, and up on subsequent incubation, it yielded diffusible brown pigmentation surrounding the colonies. The isolated bacterium was identified as Pseudomonas aeruginosaby standard biochemical tests. Mittal et al., ¹⁵ categorized bacterial pathogens as avirulent, which exhibited LD₅₀>10⁸ cfu/mL. The recovery of fish in 3-4 days post-injection period, the LD₅₀ value of >5.65×10¹⁰ cells/ fish and the categorization of Mittal et al. ¹⁵ suggested that the antagonistic bacterial strain Pseudomonas aeruginosa FARP72 is avirulent or less pathogenic.

Challenge test with pathogenic β-haemolytic Aeromonas hydrophila N10P: Clinical signs, behaviour and mortalities of the challenged fish were recorded after injection. Lethargy, abnormal behaviour, wandering around corners and erect swimming were the first signs observed in challenged Labeo rohita. The first mortality observed within 2 h post-injection. All 10 fish died where β-haemolytic Aeromonas hydrophila N₁₀P were inoculated @ 6.37x10¹⁰ cells/fish in 24 h of post-injection; whereas 9 out of 10 challenged fish died in the stock inoculated with 6.37x10⁹ cells/fish within 24 h. All the fish in third stock containing 10 fish, which were inoculated with 6.37x10⁸ cells/fish died within 48 h post-injection and 6.37x10⁷ cells/fish inoculated stock recorded two mortalities after 3^rd day. No mortalities were noticed in stocks which were used as positive control and negative control. After 15 days observation, 100% fish mortalities were observed in the stocks inoculated with 6.37x10¹⁰ cells/fish, 6.37x10⁹ cells/fish and 6.37x10⁸ cells/fish. Twenty percent (20%) mortalities were recorded in the stock where fish were inoculated with β-haemolytic Aeromonas hydrophila N₁₀P @ 6.37x10⁷ cells/fish. The lethal dose at which 50% of the experimental population die (LD₅₀) was calculated on the basis of mortalities in all test concentrations over a period of 15 days. The LD₅₀ value of β-haemolytic Aeromonas hydrophila N10P was calculated as 2.37×10⁸ cells/fish (Table 4).

According to Sarkar and Rashid ¹⁶ Aeromonas hydrophila caused 60 - 100% mortalities in all the tested fish within 2-11 days at water temperature of 30°C. Santos et al. (1991) determined, during seven days, the LD₅₀ of Aeromonas hydrophila to several fish species: Salmo trutta (2x10⁵ cells/mL), Anguilla japonica (>10⁸ cells/mL), Plecoglossus altivelis (8.6x10⁴ cells/mL), Lepomis macrochirus (>10⁸ cells/mL) and Onchorhyncus mykiss (3.2x10⁴ - 3.2x10⁸ cells/mL). Oliveira et al. ¹⁷ indicated that the 96-h LD₅₀ value of A. hydrophila to Matrinxã (Brycon amazonicus) was 6.66 x 10¹¹ cells/ mL. The LD₅₀ value of Aeromonas hydrophila was observed as 0.3× 10^8.66 cells/fish in common carp, Cyprinus carpio ¹⁸. It can be interpreted on comparing the results of the LD₅₀ value (2.37×10⁸ cells/fish) of β-haemolytic Aeromonas hydrophila N₁₀P with those of earlier studies mentioned above that the present strain can be categorized as a moderately virulent.

TABLE 4: LD₅₀ VALUE OF THE ANTAGONISTIC AND FISH PATHOGENIC BACTERIA

Pathogenicity test by cohabitation challenge: The results of the mortalities in Labeo rohita challenged withβ- haemolytic Aeromonas hydrophila N₁₀P and Pseudomonas aeruginosaFARP72 at different concentrations in cohabitation challenge are represented in Fig. 1. Clinical signs, behaviour and mortalities of the fish were recorded daily up to 30 days after challenge and cohabitation. Lethargy, abnormal behaviour, wandering around corners and vertical swimming were the first signs observed in challenged Labeo rohita. About 75% of the fish treated with β- haemolytic Aeromonas hydrophila N₁₀P suspension containing 1x10⁵ cells/ml died and only 30% mortalities were observed in fish challenged with Pseudomonas aeruginosaFARP72 at a level of 1x10⁷ cells/ml.

Significant difference existed in the mortalities of L. rohita challenged with β- haemolytic Aeromonas hydrophila N₁₀P and Pseudomonas aeruginosaFARP72 (t = 0.002; df: 2; P<0.05). Samples from the affected haemorrhagic parts of the body of freshly dead and moribund fish of the β- haemolytic Aeromonas hydrophila N₁₀P infected stock yielded typical yellow colonies on glutamate starch phenol red agar (Hi-Media, Mumbai, India) plates after 24 h of incubation at 30±2°C. The isolated bacterium was identified as Aeromonas hydrophila by standard biochemical tests.

FIGURE 1: MORTALITIES IN LABEO ROHITA BY COHIBITATION AND MODIFIED COHABITATION WITH PSEUDOMONAS AERUGINOSA FARP72 AND AEROMONAS HYDROPHILA N₁₀P

The results of the cohabitation experiments thus indicated that the disease can be reproduced under experimental conditions and that injection is not required to cause disease and mortality ¹⁹. The results of the pathogenicity trials by conventional intra-muscular injection method and cohabitation challenge showed variations, possibly due to the use of different size groups and challenge dose levels. The Labeo rohita used in the intra-muscular injection experiment were juveniles, whereas the Labeo rohita used in cohabitation challenge were fingerlings and both groups might have had different immune status.

Furthermore, the number of fish stocked in the tanks was lower in the first (n=10/tank) compared to the later (n=50/tank). This suggested that the fish mortality was more dependent on age, size, stocking density in tanks and challenge dose and all factors influence the infection pressure ²⁰. Nevertheless, all these results have proved that Pseudomonas aeruginosa FARP72 was an avirulent or less pathogenic bacterium; while the β- haemolytic Aeromonas hydrophila N₁₀P was a moderately virulent bacterium.

Biocontrol experiment: The results of the biocontrol experiment on the basis of mortalities in cohabited Labeo rohita challenged separately or together withβ- haemolytic Aeromonas hydrophila N₁₀P and Pseudomonas aeruginosaFARP72 are represented in Fig. 2.

FIGURE 2: IN-VIVO CONTROL OF AEROMONAS HYDROPHILA N₁₀P BY PSEUDOMONAS AERUGINOSA FARP72 IN COHABITATION CHALLENGE EXPERIMENT WITH LABEO ROHITA

Clinical signs, behaviour and mortalities of the challenged fish were recorded for 30 consecutive days. The fish which received a first 15 min dip in Pseudomonas aeruginosaFARP72 suspension containing 1x10⁷cells/ml followed by another dip in β- haemolytic Aeromonas hydrophila N₁₀P suspension containing 1x10⁵ cells/ml for 15 min, showed 37% mortality; while 30% mortalities were observed in fish challenged only with Pseudomonas aeruginosaFARP72 at a level of 1x10⁷ cells/ml. On the other hand, 75% of the fish died when treated only with β- haemolytic Aeromonas hydrophila N₁₀P suspension containing 1x10⁵ cells/ml.

Positive control skin wounded fish and negative control non-injured fish exhibited 19% and 9% mortalities, respectively during the same period of observation. The cumulative mortalities in Labeo rohita are depicted graphically in Fig. 3. The mortalities increased with increasing days of post-challenge. The mortality was the maximum in β- haemolytic Aeromonas hydrophila N₁₀P challenged fish, followed by mixture of Pseudomonas aeruginosaFARP72 and β- haemolytic Aeromonas hydrophila N₁₀P challenged fish, Pseudomonas aeruginosaFARP72 challenged fish, positive control and negative control fish stocks.

FIGURE 3: BIOCONTROL OF AEROMONAS HYDROPHILA N₁₀P BY PSEUDOMONAS AERUGINOSA FARP72 IN COHABITATION CHALLENGE: CUMULATIVE MORTALITY IN LABEO ROHITA

Significant differences in the mortalities were observed among the L. rohita

i)        Challenged with β- haemolytic Aeromonas hydrophila N₁₀P and Pseudomonas aeruginosaFARP72 groups (t = 0.025; df: 2; P<0.05).

ii)      Challenged with β- haemolytic Aeromonas hydrophila N₁₀P and positive control groups (t = 0.016; df: 2; P<0.05);

iii)    Challenged with Aeromonas hydrophila N₁₀P and negative control groups (t = 0.011; df: 2; P<0.05);

iv)    Challenged with Aeromonas hydrophila N₁₀P and both Pseudomonas aeruginosaFARP72 and β- haemolytic Aeromonas hydrophila N₁₀P groups (t = 0.033; df: 2; P<0.05);

v)      Positive control and negative control groups (t = 0.019; df: 2; P<0.05);

vi)    Challenged with Pseudomonas aeruginosaFARP72 and positive control groups (t = 0.039; df: 2; P<0.05),

vii)  Challenged with Pseudomonas aeruginosaFARP72 and negative control groups (t = 0.011; df: 2; P<0.05),

viii)      Challenged with both Pseudomonas aeruginosaP72 and β- haemolytic Aeromonas hydrophila N₁₀P and positive control groups (t = 0.016; df: 2; P<0.05),

ix)    Fish challenged with both Pseudomonas aeruginosaFARP72 and β- haemolytic Aeromonas hydrophila N₁₀P and negative control groups (t = 0.006; df: 2; P<0.05).

It is important to note that no significant difference existed in the mortalities of L. rohita challenged with Pseudomonas aeruginosaFARP72 and L. rohita challenged with both Pseudomonas aeruginosaFARP72 and β- haemolytic Aeromonas hydrophila N₁₀P (t = 0.167; df:2; P>0.05). It has been found that several species of Pseudomonas are pathogenic to the fish and others are non-pathogenic and can be used for control of some fish bacterial pathogens ³.

This study evaluated the efficacy of non-pathogenic antagonistic bacterium Pseudomonas aeruginosa FARP72 as biological control agent against β- haemolytic Aeromonas hydrophila N₁₀P.

As seen in Figures 2 and 3, the mortality was maximum in β- haemolytic Aeromonas hydrophila N₁₀P challenged fish, followed by mixture of Pseudomonas aeruginosa FARP72 and β- haemolytic Aeromonas hydrophila N₁₀P challenged fish, Pseudomonas aeruginosa FARP72 challenged fish, positive control and negative control fish stocks.

Positive control skin wounded fish and negative control non-injured fish exhibited 19% and 9% mortalities, respectively. In positive control all the skin wounded fish died, may be due to secondary infection. The observed fish mortality in negative control tanks could be attributed to the fluctuations in environmental parameters as a result of monsoon rains during the experimental period.

Several studies reported that the in-vitro inhibition effects of bacterial pathogens are difficult to reproduce in in-vivo conditions ^{21, 22}. However, the present work demonstrated that Pseudomonas aeruginosa FARP72 was capable of inhibiting human and fish pathogens in-vitro and fish pathogenic Aeromonas hydrophila in-vivo possibly through the production of antibacterial compounds as demonstrated in this study and competitive exclusion principles. Siderophore mediated competition for iron may also explain the inhibitory activity of the antagonistic strain ²³. The antimicrobial activity of Pseudomonas species has been attributed to several antibiotic-like substances such as phenazines ²⁴ and non-nitrogen-containing compounds such as 2, 4-diacetylphloroglucinol ²⁵.

A Pseudomonas sp. isolated from a brackish water lagoon showed significant probiotic activity against a number of shrimp pathogenic vibrios, while its safety in a mammalian system was also found satisfactory ²⁷. In another study, Korkea-aho et al ²⁶recorded lower levels of mortalities in fish fed first with Pseudomonas M174 and subsequently challenged with Flavobacterium psychrophilum.

CONCLUSIONS: The use of antagonistic bacteria is an importantmanagement tool, but its efficiency depends on understanding the nature of competition betweenspecies or strains. It is clear from the results of the studies that colonization of the antagonistic bacterium Pseudomonas aeruginosa FARP72 on fish such as Labeo rohita would help reducing the attachment of β- haemolytic Aeromonas hydrophila and development of infection and disease outbreaks. Still there is a need to continue to screen new antagonistic strains from local aquaculture rearing species to suit the specific requirement. The results of the present study would provide the basis for future research on biological control of fish diseases and its application in aquaculture.

ACKNOWLEDGEMENT: The author desires to express sincere thanks to the Calcutta National Medical College and Hospital, Calcutta; Fisheries College and Research Institute, Tamilnadu Fisheries University; Shrimp Disease Diagnostic Laboratory, Tamil Nadu University of Veterinary and Animal Sciences; Department of Fish Processing Technology, Faculty of Fishery Sciences, West Bengal University of Animal and Fishery Sciences, for helping with human and fish pathogenic bacterial isolates and also to the Vice-Chancellor, West Bengal University of Animal and Fishery Sciences, Kolkata for providing necessary infrastructure facility to carry out the work.

REFERENCES

1. FAO, Global Aquaculture Production Statistics for the year 2011 (online), 2013. ftp://ftp.fao.org/FI/news/GlobalAquaculture Production Statistics 2011.
2. Nogales B, Lanfranconi MP, Pina-Villalonga JM and Bosch R: Anthropogenic perturbations in marine microbial communities. FEMS Microbiology Reviews 2011; 35: 275 –298.
3. Austin B and Austin DA: Bacterial Fish Pathogens: Diseases in Farmed and Wild Fish. Chichester, UK, Edition 4, 2007.
4. John N and Abdullah MH: Distribution, Extracellular Virulence Factors and Drug Resistance of Motile Aeromonads in Fresh Water Ornamental Fishes and Associated Carriage Water. International Journal of Aquaculture 2013; 3(17): 92-100.
5. Gomez-Gil B, Roque A and Turnbull JF: The use, selection of probiotic bacteria for use in the culture of larval aquatic organisms. Aquaculture 2000; 191: 259-270.
6. Robertson PO, Dowd C, Burrells C, Williams P and Austin B: Use of Carnobacterium sp. as a probiotic for Atlantic salmon (Salmo salar L.), rainbow trout (Oncorhynchus mykiss, Walbaum). Aquaculture 2000; 185: 235-243.
7. Cao H, He S, Wei R, Diong M and Lu L: Bacillus amyloliquefaciens G1: A Potential Antagonistic Bacterium against Eel-Pathogenic Aeromonas hydrophila. Evidence based complementary and alternative medicine 2011; 3(2):824-834.
8. Austin B, Baudet E and Stobie M: Inhibition of bacterial fish pathogens by Bergey’s Manual of Determinative Bacteriology http://www.uiweb.uidaho.edu/microbiology/250/IDFlowcharts.pdf. 1992.
9. Lemos ML, Toranzo AE and Barja JL: Antibiotic activity of epiphytic bacteria isolated from intertidal seaweeds. Microbial Ecology 1985; 11, 149-163.
10. Nakamura A., Takahashi KG, Mori K, Vibriostatic bacteria isolated from rearing seawater of oyster brood stock: potentiality as biocontrol agents for Vibriosis in oyster larvae. Fish Pathology 1999; 34(3): 139-144.
11. Miles AA, Misra SS, Irwin JO, The estimation of the bactericidal power of the blood. The Journal of Hygiene 1938; 38 (6): 732–749.
12. Iqbal MM, Tajima K, Ezura Y, Phenotypic and genotypic identification of motile aeromonads isolated from fish with epizootic ulcerative syndrome in Southeast Asian countries. Bulletin of the Japanese Society of Scientific Fisheries 1999; 50(2): 93-100.
13. Reed LJ, Muench H: A simple method of estimating fifty percent end points. American Journal of Hygiene 1938; 27: 493–497.
14. Madetoja J, Nyman P, Wiklund T, Flavobacterium psychrophilum, invasion into, shedding by rainbow trout Oncorhynchus mykiss. Diseases of Aquatic Organisms 2000; 43: 27-38.
15. Mittal KR, Lalonde KK, Leblanc D, Olivier G, Lallier R, Aeromonas hydrophila in rainbow trout: relation between virulence, surface characteristic. Canadian Journal of Microbiology 1980; 26: 1501–1503.
16. Sarkar MJA, Rashid MM, Pathogenicity of the bacterial isolate Aeromonas hydrophila to catfishes, carps and perch. Journal of Bangladesh Agriculture University 2012; 10(1):157–161.
17. Oliveira SRD, Souza RTYBPD, Brasil EM, Andrade JIAD, Nunes EDSS, Ono EA, Affonso EG: LD50 of the bacteria Aeromonas hydrophila to matrinxã, Brycon amazonicus. Acta Amazonica 2011; 41(2): 321-326.
18. Alsaphar SAA, Al-Faragi JKH, Detection and study of the experimental infection of Aeromonas strain in the common carp (Cyprinus carpio L.). Iraqi Journal of Veterinary Medicine 2012; 36 (2): 222– 230.
19. Lunder T, Evensen O, Holstad G, Hastein T, Winter ulcer' in the Atlantic salmon Salmo salar. Pathological and bacteriological investigations, transmission experiments. Diseases of Aquatic Organisms 1995; 23: 39-49.
20. Abd-el-Rhman AM, Khattab YAE, Adel ME, Shalaby AME, Micrococcus luteus and Pseudomonas species as probiotics for promoting the growth performance and health of Nile tilapia, Oreochromis niloticus. Fish and Shellfish Immunology 2009; 27: 175-180.
21. Hurdle JG, O’Neill, AJ, Chopra I, Lee RE, Targeting bacterial membrane function: an underexploited mechanism for treating persistent infections. Nature Reviews Microbiology 2011; 9(1): 62–75.
22. Gram L, Inhibitory effect against pathogenic, spoilage bacteria of Pseudomonas strains isolated from spoiled and fresh fish. Applied and Environmental Microbiology 1993; 59: 2197–2203.
23. Hamdan H, Weller DM, Thomashow LS, Relative importance of fluorescent siderophores and other factors in biological control of Gaeumannomyces graminis var. Tritici by Pseudomonas fluroscens strains 2-79. Applied and Environmental Microbiology 1991; 57: 3270-3277.
24. Shanahan PD, Sullivan JO, Simpson, P, Glennon JD, Gara FO, Isolation of 2,4-diacetylphloroglucinol from a fluorescent pseudomonad and investigation of physiological parameters influencing its production. Applied and Environmental Microbiology 1992; 58: 353–358.
25. Irianto A, Austin B, Use of dead probiotic cells to control furunculosis in rainbow trout, Oncorhynchus mykiss (Walbaum). Journal of Fish Diseases 2000; 26 (1): 59–62.
26. Korkea-aho TL, Heikkinen J, Thompson KD, VonWright A., Austin B, Pseudomonas sp. M174 inhibits the fish pathogen Flavobacterium psychrophilum. Journal of Applied Microbiology 2011; 213-215.
    

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    How to cite this article:

    Hoque F: Biocontrol  of  β - Haemolytic Aeromonas  hydrophila infection  in  Labeo  rohita  using  antagonistic bacterium  Pseudomonas  aeruginosa  FARP72. Int J Pharm Sci Res 2014; 5(2): 490-01.doi: 10.13040/IJPSR.0975-8232.5(2).490-01

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